1. Field of the Invention
The present invention is directed toward a method and a device for treating surfaces with reactive particles generated in a glow discharge plasma, in particular for treating surfaces that are highly sensitive to thermal loading. The plasma used in the inventive method and device is a so-called one atmosphere glow discharge plasma, i.e. a plasma sustained in a gas at pressures in the range of ambient pressure (ca. 0.5 to 1.5 bar or 50,000 to 150,000 Pa).
2. Description of Related Art
Glow discharge plasmas are sustained in a gas, for example with the aid of an alternating electric field of a suitable voltage and frequency by which the gas is partially ionized to contain ions and electrons in addition to neutral atoms, molecules and radicals. The glow discharge plasma is usually sustained between two plate electrodes being correspondingly energized and having two substantially parallel electrode faces between which the plasma is located.
Surfaces are treated with reactive particles produced in a glow discharge plasma in order to, for example, change the surface energy (change of wettability). sterilize the surface, etch the surface. or deposit a material layer on the surface. The gas or gas mixture to be broken down in the plasma and possibly the plasma parameters are selected according to the desired effect of the plasma treatment.
Usually, the gas or gas mixture to be broken down in the plasma for the above-mentioned surface treatment processes has a reduced pressure in the order of 1 mbar (100 Pa) or less and the surface to be treated is either exposed to the plasma directly by being positioned between the two electrodes (direct plasma treatment) or it is positioned away from the plasma and the gas is made to flow from between the electrodes onto the surface to be treated (remote plasma treatment). Remote plasma treatment is chosen, for example, for geometric reasons, for shielding the surface to be treated from the light emission in the plasma, or for surface treatment with a selection only of the reactive particles occurring in the plasma (selection of longer living particles)
Reduced pressure glow discharge plasma has a relatively low power density and a high uniformity. Therefore, it is highly suited to treat surfaces sensitive to thermal loading. However, as the pressures used are very low, devices suitable for such surface treatment need to be vacuum tight and are correspondingly complicated and costly, particularly if large surfaces such as, for instance, webs of film are to be handled or if large amounts of items are to be handled with relatively short cycle times (without venting of a plasma chamber in each treatment cycle).
One exemplified application of surface treatment with the aid of a plasma is a process in which thin plastic film (e.g., low density polyethylene, polypropylene. amorphous or biaxial oriented polyester, cast or biaxially oriented polyamide with a film thickness of e.g. 80 μm) is coated with a layer of silicon oxide in order to reduce the gas permeability of the film (barrier layer). The process is a plasma enhanced vapor deposition process and the gas mixture contains beside oxygen and an inert gas such as a vaporized organosilicon compound. Similar processes are used for depositing barrier layers of silicon nitride, silicon carbide, or carbon (amorphous or crystalline). For the coating with carbon the treatment gas mixture contains light hydrocarbons such as methane, ethylene, ethane, etc.
Conventionally, for carrying out such processes a rotating drum serving as one electrode and a counter electrode with a correspondingly concave electrode face are arranged in a vacuum chamber and the plastic web is advanced between the two electrodes being positioned on the rotating drum while the gas mixture is fed to the space between the two electrodes. Usually, both a feed roll of film and a web roll for taking up the coated web are also positioned within the vacuum chamber.
It is also known that glow discharge plasmas can be sustained in gases at substantially ambient pressure. Plasma sustained in air of ambient pressure is used for producing ozone For this process it does not matter that such one atmosphere plasmas are highly filamentary which, however, renders them unsuitable for surface treatment, in particular for treatment of sensitive surfaces. In U.S. Pat. Nos. 5,387,842; 5,456,972; 5,403,453; and, 5,414,324, the disclosures of which are expressly incorporated herein in their entireties, the University of Tennessee Research Corporation discloses methods for producing more uniform one atmosphere glow discharge plasmas (plasmas sustained in a gas of substantially ambient pressure). The uniformity of these plasmas is achieved by carefully matching voltage and frequency to the character and the pressure of the used gas or gas mixture. According to the disclosure, such plasmas are suitable for treating surfaces by positioning the surface to be treated between two correspondingly energized plate electrodes.
However, it shows that the uniformity of such one atmosphere plasmas is very sensitive to small parameter changes (power, frequency, gas composition, gas pressure, geometry etc.) such that it is hardly possible to sustain such a plasma in a continuously working system over a longer period of time strictly without filaments. According to a later publication of the University of Tennessee Research Corporation (WO-99/40758). “it becomes difficult to prevent occasional plasma filaments from developing at the edges of the workpiece, the electrode edges, or the edges of the web when a workpiece is directly exposed to the plasma”, i.e. when the workpiece is positioned between the two plate electrodes. Such occasional filaments become particularly troublesome in the above-named barrier coating application in which each filament punches a hole into the plastic film resulting in a hole in the produced barrier coating also and, therefore, resulting in a leak which, even if microscopically small, outweighs the effect of the barrier layer over a large area of the coated surface. Therefore, it seems hardly possible to achieve top quality barrier properties in a direct plasma treatment using a one atmosphere plasma.
For this reason, the named publication WO-99/40758 suggests to treat surfaces with reactive particles generated in a one atmosphere glow discharge plasma in remote plasma processes, in particular in the case of thin and heat-sensitive plastic films and fabrics. However, this again renders a device for carrying out the process more complicated and restricts the availability of some of the reactive particles, in particular the availability of very short lived particles.